Field of the Invention
The present invention relates to an in-situ method and system for measuring wellbore pressures in a formation. More specifically, a pressure gauge is arranged to be permanently cemented in place outside of a wellbore conduit, and pressure measurements signals representing the formation pressure are sent to a control unit.
Description of Prior Art
Different technologies can be applied for measurement of the pressure in the formation surrounding the wellbore, but in general some type of a pressure gauge is arranged in the formation, or in contact with the formation.
International patent publication WO2007/056121 A1 discloses a method for monitoring formation pressure, where the gauge is shot from a gun attached to the wellbore conduit through the cement and into the formation.
International publication WO2012073145 A1 discloses a method for measuring pressure in an underground formation by establishing a flowline and a piston to suction fluid into a test chamber.
International publication WO2013052996 discloses a method for installing a pressure transducer in a borehole, where a fluid connection between the transducer and the sensor is established through the cement.
U.S. Pat. No. 5,467,823 shows a method and apparatus of monitoring subsurface formations by means of at least one sensor responsive to a parameter related to fluids, comprising the steps of: lowering the sensor into the well to a depth level corresponding to the reservoir; fixedly positioning the sensor at the depth while isolating the section of the well where the sensor is located from the rest of the well and providing fluid communication between the sensor and the reservoir by perforating the cement.
In general all permanent pressure gauges have a sensor, a fluid fill, and a process isolation system. The sensor is often a quartz crystal resonator sensor. The process isolation system protects the oil around the sensor itself, as this needs to be in an oil filled and inert medium to measure the pressure in the fluid. The isolation system may typically be established by a bellows or using a diaphragm or by one or more relatively large oil volume oil chambers in series separated by a buffer tube system.
The negative side of using a diaphragm is that a relatively wide area diaphragm is needed to provide effective and sufficient volume compensation of the oil fill surrounding the sensor. In turn, a larger area diaphragm is vulnerable to damage and overexposure of its dynamic range.
Buffer tubes are coiled pieces of tubing that are attached to the sensor port. The buffer tube serves as a mechanical isolator to prevent shock or vibration from being transmitted directly to the sensor. However, buffer tubes in series with one or more coupled oil chambers is not really an isolation system as oil is in a continuous contact from the outside and inward to the sensor. Another related problem is that the buffer tubes may clog up with time.
U.S. Pat. No. 4,453,401 shows a system for measuring transient pore water pressure in the ground utilizes a probe member with an arrangement of a pressure sensor and a soil stress isolation filter. The probe member has a body portion with a hollow cavity defined therein. The pressure sensor in the form of a ceramic transducer is mounted in the cavity.
The use of bellows are known from prior art. However, in a traditional pressure gauge configuration, the pressure port of the pressure gauge housing is open to the environment. In turn, this exposes the bellows to the fluids of the surroundings without being filtered. This typically lead to deposition of sediments in the chamber housing the bellows, which inhibits it freedom to move with time or in worst case becoming non-functional as an elastic element transferring the pressure from the outside to the inside. The latter is typically the case if the sensor is placed in a location that is being cemented. Cement will fill the housing surrounding the bellows and as it hardens the pressure gauge will be isolated and disabled to see the pressure change on the outside wellbore or formation, as the bellows is no longer able to work as an elastic element.